1. Field of the Invention
The present invention relates to a bistatic azimuth detection system and a detection method thereof, and in particular, relates to a bistatic azimuth detection system and a detection method which detect an azimuth of a target by receiving and analyzing a reflective sound, obtained by a sound which is radiated from sound source equipment and is reflected by the target in the sea, with one or more wave receiving equipment in positions different from that of the sound source equipment.
2. Description of the Related Art
FIG. 9 shows an explanatory diagram of an example of a conventional bistatic azimuth detection method. In FIG. 9, a sound wave transmitted by a sound source unit 30 is reflected by a target 31 in the sea, and the reflective sound is received in a wave receiving unit 32 installed in a position different from the sound source unit 30. The wave receiving unit 32 converts the received reflective sound into a reflective acoustic signal which is an electric signal, and outputs it to a processing unit 33. The processing unit 33 detects an azimuth, having a highest sound level, as an arrival azimuth to a reference axis of the wave receiving unit 32 by analyzing a sound level, which the wave receiving unit 32 receives for every unit azimuth, on the basis of the inputted reflective acoustic signal.
In FIG. 9, θ shows the above-described arrival azimuth. Moreover, the wave receiving unit 32 has a compass and detects a magnetic north azimuth of the reference axis of the wave receiving unit 32 to output it to the processing unit 33. In FIG. 9, φ shows the magnetic north azimuth of the above-described reference axis. Then, the processing unit 33 adds the magnetic north azimuth φ of the reference axis to the arrival azimuth θ of the reflective sound from the target 31, and detects the result as a target azimuth of the target 31.
In addition, bistatic azimuth detection equipment which automates target echo detection and target position localization is also known conventionally (for example, refer to Japanese Patent Laid-Open No. 2001-296359). The conventional bistatic azimuth detection equipment mentioned in Japanese Patent Laid-Open No. 2001-296359 comprises a signal processing unit which performs the processing of removing a narrow-band signal whose frequency is stable in time, from an acoustic signal received by a receiving unit, and emphasizing a target echo, a primary detection processing unit which extracts a signal appropriate for a target echo by setting a threshold, a secondary detection processing unit which classifies the result of primary detection processing into categories, such as a target echo, a direct wave, a submarine reflective wave, and sea noise, by a neural network, and detects the target echo, and a target position localization processing unit which calculates arrival time difference between the detected target echo and direct wave, and localizes a target existence zone by ellipse drawing to detect a target position from an intersection with a signal arrival azimuth.
Further, a signal detection system is known conventionally, the signal detection system which can distinguish whether a received echo is one from a true target or one reflected on a sea surface or a sea bottom. The signal detection system includes a plurality of directive passive sonobuoys each receiving an echo of a sound wave which a sound source sonobuoy radiates underwater, calculates an existence zone of the target, which is a sound source of echoes which they receive (including a sea surface and a sea bottom), on a two-dimensional coordinate plane for every directive passive sonobuoy from the positional relation and propagation time between the sound source sonobuoy and each of directive passive sonobuoys, cumulates an echo level for every target existence zone, and compares the echo cumulation level with a threshold level (for example, refer to Japanese Patent Laid-Open No. 7-294640).
However, the conventional bistatic azimuth detection method shown in FIG. 9 cannot perform highly accurate azimuth detection due to error factors such as the accuracy of a compass, and an earth magnetism deviation. In addition, since an azimuth detection function such as a compass is indispensable, a mechanism is enlarged and cost increases.
In addition, since the conventional equipment mentioned in Japanese Patent Laid-Open No. 2001-296359 performs classification into categories, such as a target echo, a direct wave, a submarine reflective sound, and sea noise, by a neural network, and detects the target echo, a load of a computer is large. The configuration of equipment is also complicated and expensive. Moreover, since it determines a magnetic north azimuth with a compass and estimates an existence position of a target by using an arrival azimuth of the target, it is also not possible to perform highly accurate azimuth detection due to the above-described error factors such as the accuracy of a compass, and an earth magnetism deviation.
Furthermore, since the conventional system mentioned in Japanese Patent Laid-Open No. 7-294640 calculates an existence azimuth of a target by using directional characteristics of three directive passive sonobuoys, three passive sonobuoys are required. Since each passive sonobuoy has a compass and detects a magnetic north direction, it is not possible to perform highly accurate azimuth detection due to the above-described error factors such as the accuracy of a compass, and an earth magnetism deviation.